The catalyzed isomerization of alpha-pinene to beta-pinene to which this invention is directed is disclosed in U.S. Pat. No. 3,278,623, the disclosure of which is expressly incorporated herein by reference. The isomerization is carried out in the presence of a transitory hydrogen acceptor catalyst using hydrogen gas as a co-catalyst. A preferred such hydrogen acceptor catalyst is a Group VIII metal on an alumina support. Such isomerization process is acutely acid sensitive as acidic conditions in the process can deactivate the otherwise active Group VIII catalyst and will cause formation of undesirable by-products (such as camphene, cymene, and limonene). Formation of such by-products (hereinafter referred to as acidic or acid by-products) during the alpha-pinene isomerization process decreases the selectivity of the alumina-supported Group VIII catalyst for beta-pinene formation.
Commercially prepared alumina-supported Group VIII catalysts can have residual anions thereon, which anions can form acid in the presence of hydrogen. Such commercial preparation generally comprises treating alumina with an aqueous salt solution of the Group VIII catalyst followed by drying and a subsequent reduction in a basic hydrazine solution or in flowing hydrogen gas. Residual anions remain on the alumina supported Group VIII catalyst even after the reduction step. These residual anions can form acid when contacted with the co-catalyst hydrogen gas during the isomerization process which can cause the aforementioned by-product formation.
The Group VIII catalyst salts can include the salts of chlorine, bromine iodine, fluorine, nitrate, sulfate and the like. The chloride salts of the Group VIII catalyst are commonly used in such commercial preparations and this invention will be described with reference to chloride salt preparation of the alumina-supported Group VIII catalyst.
Most of the various forms of alumina readily hydrate to form acid sites (hydroxyl groups) thereon which hydroxyl groups can serve as a source of protons, the alumina displaying acidic functionalities thereby. The residual chloride ions further can exchange with such hydroxyl groups which exchange intensifies the electronegativity of the acid sites. Heretofore, it has been proposed to neutralize these acid sites of an alumina catalyst (as distinguished from an alumina-supported catalyst) by treating the alumina with amines. Herman Pines and C. N. Pillai in "Alumina: Catalyst and Support X. Modification of Alumina by Bases. Mechanism of Dehydration of Menthol and Neomenthol," Journal American Chemical Society, Volume 83, pp. 3070-3274 (1961) describe treating an alumina catalyst with amines such as ammonia, trimethylamine, and pyridine in order to neutralize the acid sites on such alumina catalysts. However, when the alumina serves as a support for a catalyst, such as elemental palladium, amines deactivate the palladium catalyst for the alpha-pinene isomerization process.
For purposes of this application, selectivity is measured as a function of the acid by-products formed during the isomerization process. By suppressing such acid by-products, selectivity of the supported catalyst is increased. Selectivity is calculated as follows: ##EQU1## wherein % alpha-pinene totally converted includes beta-pinene and acid by-products. It is readily apparent that as the percent of acidic by-products is decreased, the selectivity approaches 100%.
Acid by-products in addition cause difficulty in effecting a clean separation of the beta-pinene from unconverted alpha-pinene and also decrease the amount of alpha-pinene available for recycle as well as contaminate such alpha-pinene recycle.